The main difference between cyst and oocyst is that cyst is the dormant stage of bacteria or protozoa that ensure survival during unfavorable environmental conditions, whereas oocyst is a stage within the life cycle of protozoa that contains a zygote in it.
Microorganisms take different forms during their life cycles to face challenges and ensure their survival within the ecosystem. Cyst and oocyst are two such structures that ensure the survival of microorganisms like bacteria and protozoa.
Key Areas Covered
What is a Cyst
Cyst is a dormant stage in the life cycle of a microorganism that ensures its survival during unfavorable environmental conditions. These unfavorable environmental conditions include high temperatures, lack of moisture, extremely cold weather, lack of essential nutrients, unfavorable toxic chemicals, etc. Until the environmental conditions become favorable or return to normal, cysts ensure the survival and existence of the microorganism. Cysts are especially present in microorganisms such as bacteria and protozoa. This is an adaptive strategy of organisms during the process of evolution.
It’s important to note that a cyst is not a reproductive cell. It has a thick cyst wall. The composition of this cyst wall varies in cysts of different species of microorganisms. In bacteria, the cyst wall is thick due to the presence of peptidoglycan layers, but in protozoans, the cyst wall thickness is due to the presence of chitin.
Laval stages of internal parasites also get enclosed within a cyst. We call this process encystment. This encystment also helps to move from one host to another or to get dispersed to a favorable environment. When the cyst moves to a favorable environment, they undergo a process called excystation. In excystation, the wall of the cyst ruptures.
Cyst formation in the encystment occurs due to the thickening of the cell wall and cytoplasmic contractions. Bacterial cysts are different from endospores depending on their formation.
What is an Oocyst
Oocyst is a cell in the life cycle of protozoa that contains a zygote within it. Oocysts are an important feature in the life cycle of some species within the phylum Apicomplexa, a group of parasitic single-celled organisms that are spore-forming and typically intracellular. These protozoans are characterized by the presence of a specialized organelle called the apical complex. While not all members of this phylum have the same variety of stages in their life cycle, oocysts are a common feature that serves as the site of zygote development.
One example of a species with oocysts in its life cycle is Toxoplasma gondii, which is the causative agent of toxoplasmosis. The life cycle of T. gondii involves several stages, including bradyzoites, tachyzoites, and oocysts. The bradyzoites may give rise to either tachyzoite or gametocytes. Gametocytes are gamete-forming cells. The male gamete, called the microgamete, is smaller and flagellated, while the female gametocyte gives rise to a larger and non-flagellated macrogamete. During fertilization, the microgamete and macrogamete fuse together to form a zygote that resides inside the oocyst. Hence, an oocyte is a structure with thick cell walls that can be observed in the life cycle of protozoa that form zygotes.
When the conditions are favorable, the zygote starts to develop within the oocyst. When the zygote develops, the oocyst becomes infective. Toxoplasmosis develops in a host body due to the ingestion of an oocyst in its infective stage. Oocysts will release bradyzoites in the gut region and in the stomach of the host body, which in turn, initiate the lifecycle of Toxoplasma gondii.
Additionally, Cryptosporidium, Isospora, and Eimeria are the other organisms that produce an oocyst in their life cycle.
Difference Between Cyst and Oocyst
A cyst is the dormant stage of bacteria or protozoa that ensures survival during unfavorable environmental conditions, whereas an oocyst is a stage within the life cycle of protozoa that contains a zygote in it.
Cysts are a form of protective structures formed by some bacteria, protozoa, and fungi to survive unfavorable environmental conditions, while oocysts are a type of reproductive structure formed by some protozoa to aid in transmission to new hosts.
A cyst is not a reproductive cell, while an oocyst is a reproductive cell.
Microorganisms take the shape of different forms and structures in their life cycles to ensure their survival in the long term. Two such structures are cysts and oocysts. The key difference between cyst and oocyst is that cyst is the dormant stage of bacteria or protozoa that ensure survival during unfavorable environmental conditions, whereas an oocyst is a stage within the life cycle of protozoa that contains a zygote in it.
1. “Dynamics of Toxoplasma gondii Oocyst Phagocytosis by Macrophages.” Dynamics of Toxoplasma gondii Oocyst Phagocytosis by Macrophages.
1. “Life cycle in symbiotic and cyst-forming Acantharia” By Johan Decelle, Patrick Martin, Katsiaryna Paborstava, David W. Pond, Geraint Tarling, Frédéric Mahé, Colomban de Vargas, Richard Lampitt and Fabrice Not –  doi:10.1371/journal.pone.0053598 (CC BY-SA 4.0) via Commons Wikimedia
2. “Toxoplasma gondii oocyst” By Servier Medical Art – (CC BY 2.0) via Commons Wikimedia